Exhaust gas discharged from a Diesel engine is passed through a trap disposed in an exhaust passage of the engine. Soot and other particulates contained in the exhaust gas are collected by a collecting material in the form of a porous ceramic substance packed in the trap. The particulates accumulate in the trap resulting in an increased impedance in the trap, and a drop in the engine output. Therefore, at the time when the particulates have accumulated in a certain quantity, it is necessary to burn off the particulates thereby regenerating the trap. For the purpose of regeneration of the trap, an electric heater is located adjacent to the upstream end of the collecting material. The heater raises the temperature of the exhaust gas to a temperature sufficient to, so ignite the particulates, which are then burnt off by the high temperature exhaust gas.
In the prior art manner of trap regeneration described above, the electric heater requires an extremely large capacity if the temperature of the entire volume of the exhaust gas is to be raised. Therefore, it has been a common practice to bypass part of the exhaust gas during the process of regeneration so as to reduce the quantity of the exhaust gas flowing into the trap.
However, when the temperature of the exhaust gas flowing into the trap is low ( for example, about 150.degree. C.), ignition and burning of the particulates is not immediately started due to the relatively large thermal capacity of the trap. The time for supply of current to the electric heater is thus correspondingly extended, with the result that the battery may be completely exhausted. Further, because the trap itself is exposed to exhaust gas whose temperature has been raised by the electric heater, a large thermal stress occurs in the trap leading to damage.
As described above, various drawbacks are involved, and improvements for obviating those drawbacks have been demanded.